I know its not cool to ask what things are without posting a pict but I think I can get an answer. I have a metal box a little bigger than a deck of cards behind dash on passenger side with one plug. Which I think is the ecu? Next to that is a yellow plastic box about same size as first mentioned that says vss so I am assuming thats what it is. Neither of those will drop out as mentioned before. (The metal box one has a metal bracket at top that needs to be unbolted so it can drop out).

Now the box in question is a metal one closes to the passenger door that did drop down ( I didnt want it to). Its probley twice as big as the others and has two wire plugs going into it. Alot of wires I think all of them coming from engine. It sits in a big plastic tray, but is held by a strip of velcro. Any how Is the big metal box the ecm, or the little metal box? What is what. Its a 91 RS (TBI 305 LO3) AUTO. 700r4. Thanks, im trying to trouble shoot why my tach/speedo doesnt work. I have to take things apart and see it before I can understand any info about it.

 

The big metal box with lots of wires and 2 plugs under the dash on the pass side is the computer for the engine. Dont think that would cause you speedo or tach not to work. The VSS is the vehicle speed sensor. That would cause the speedo not to work if its not working properly. I guess thats what that box would be. Not forsure what the small metal box would be. My iroc is an 89 and the systems in my car are setup a little different than a 91. My VSS is on the back of my instrument cluster. Other members should be able to chime in.

 

As far as spedo/tach I can think of a few wires to check before pulling the comp for whatever reason.Check for damage to a white wire that comes off the ignition coil and runs to the ins cluster. Not the one that goes back to the Dist. Also there are two wires gray and brown/white that run from the ECM check those as well.